A digital filter is integrated, miniaturized and manufactured with low cost and high reliability, and so have much advantage compared with an analog filter. Specially, application field of the digital filter has been increased according as communication velocity is realized with high speed and an amount of communication is augmented, and the digital filter has been employed in devices such as a transmitter and a receiver of a baseband module in a mobile communication system.
The digital filter is generally divided into a finite impulse response filter and an infinite impulse response filter.
The finite impulse response filter uses characteristic that impulse response has a finite length when the impulse response is inputted to the filter. This is because the finite impulse response filter does not use any feedback.
The finite impulse response filter not using the feedback does not need feedback loop, and thus stability of the filter is guaranteed. Specially, since the finite impulse response filter satisfies linear phase characteristic, the finite impulse response filter has been widely used in applications such as waveform transmission, etc. However, in case that the finite impulse response filter will realize approximately the same amplitude as the infinite impulse response filter, order of the finite impulse response filter is more increased. As a result, the finite impulse response filter is more loaded in view of hardware including an adder and a multiplier.
The finite impulse response filter may be designed through a design method in a frequency domain and a design method in a time domain, and a window function method and a frequency sampling method, etc. are mainly used when the finite impulse response filter is designed through the design method in the frequency domain.
The design of the filter in the time domain is simpler than that in the frequency domain because impulse response in the design of the filter in the time domain corresponds to coefficients of the filter. A linear programming is widely known as a method of approximating a transfer function, and an optimal solution may be calculated through finite calculation in case that the optimal solution exists.
Specially, output finitude of the finite impulse response filter allows to omit a calculation process of not generating decimated output or a calculation process of generating a predictable value in an interpolated output, and thus this is calculatedly efficient when performing interpolation or decimation so as to increase or reduce multirate application, e.g. sampling rate of a signal.
Since a number of taps and coefficients, etc. of the finite impulse response filter are fixed, it is difficult to reconfigurable pass band characteristic in case that the finite impulse response filter is designed to realize certain pass band.
FIG. 7 is a view illustrating example of a conventional filter for reconfiguring frequency through coefficient change of a filter.
Referring to FIG. 7, the conventional frequency reconfigurable filter includes a coefficient storage section 100, a multiplexer 102, a multiplier 104 and an adder 106. The coefficient storage section 100 stores coefficient information for plural pass bands.
Coefficient information is extracted in accordance with a user's band selection and the extracted coefficient information is provided to the multiplexer 102 to generate impulse response corresponding to a selected band. The generated impulse response is applied to inputted function x(n), and so an output signal y(n) is generated by filtering the inputted function x(n).
The conventional reconfiguration method has limitation of a reconfigurable band. Only coefficient of the filter is changed though the frequency is reconfigured, and thus it is difficult to reconfigure properly the frequency.
It is impossible to reconfigure frequency in frequency band except frequency corresponding to pre-stored coefficients of the filter. Accordingly, the coefficients of the filter need to be stored one by one so as to reconfigure the frequency, and thus the filter should have a memory for storing much information.
Recently, an equalizer has been generally used in a hearing aid and playing of voice source, etc., reconfiguration of a pass band in accordance with the user's favor is one of important functions of the equalizer, and the frequency reconfiguration of the filter is essentially required for provision of an improved equalizer.